This invention relates to a method and apparatus for controlling an atmosphere, such as the atmosphere in a fruit store.
Considering apples as an example, apples are harvested over only a few weeks in the year but must be available to the retail market all year round. Apples may be satisfactorily stored at low temperature in a modified atmosphere. Typically, UK grown Cox's Orange Pippin is stored for 7-8 months at 3.5.degree.-4.0.degree. C., in a nitrogen atmosphere containing 1 to 11/4% O.sub.2 and containing under 1% CO.sub.2. Apples respire even after they are picked, consuming O.sub.2 and producing CO.sub.2. Hence, in a sealed store, this O.sub.2 concentration is reached by itself (after about 8 days) and is maintained simply by the controlled admission of air (21% O.sub.2); the problem is to remove excess CO.sub.2.
For removing CO.sub.2, the most commonly used scrubbing technique (in the UK) is the addition to the store of dry, bagged, hydrated lime (Ca(OH).sub.2) which absorbs CO.sub.2 by chemical reaction. This is simple, reliable, and requires no capital outlay. However, the running costs are high, about 300 per year for a 100 tonne fruit store. Additionally, the labour costs are high, the lime being messy and awkward to handle. Furthermore, the entire annual demand for lime for this purpose arises over a few weeks, which makes it unattractive for manufacturers to cater specially for this demand.
A common, more convenient, alternative to the use of lime is a mechanical activated carbon adsorber. This works by passing store atmosphere through a bed containing activated carbon so that CO.sub.2 is adsorbed and the remaining gas returned to the store. Typically after 5-10 minutes, the activated carbon becomes saturated with CO.sub.2. It is then regenerated by passing fresh air through the bed, whereupon CO.sub.2 is desorbed into the air stream. Once the bed is free of CO.sub.2 it is ready for a further adsorption phase.
This simple adsorber suffers the important shortcoming that, following regeneration, the bed is left full of air, which in the ensuing adsorption phase is discharged into the store. In this way, oxygen is repeatedly discharged into the store, often at a faster rate than the fruit consumes it, so that the optimum oxygen concentration becomes exceeded.
To reduce this shortcoming, most scrubber manufacturers adopt a valve control sequence whereby, between regenerating the bed and the next adsorption phase, the bed is briefly purged with store atmosphere (which is then vented to exhaust) to remove excess oxygen from the bed. An inverse sequence is organised between adsorption and regeneration so that store atmosphere remaining in the bed (with its valuable low oxygen concentration) is not expelled to waste.
These sequences do indeed decrease the mass of oxygen added to the store via the scrubber, but at the cost of subjecting the store to a slight cyclic vacuum and overpressure. Hence if the store is not absolutely gas-tight, air gain or store-atmosphere loss will occur through leaks in the structure of the store and around doors and hatches.
Scrubbers operating in this way are satisfactory for use at 1% CO.sub.2 +11/4% O.sub.2, only if they are well maintained. In one trial, such a scrubber, operated with great care, did keep the CO.sub.2 down to 0.8%. In practice, however, the performance of many commercial scrubbers is such that higher CO.sub.2 concentrations have to be accepted in order to maintain low oxygen concentrations.